This invention relates to a control for an electric toaster for obtaining a desired toast color and in particular to a control which will vary the length of the toasting cycle in accord with variations in the voltage supply to the toaster during the toast cycle or in accord with temperature increases in the cavity as a consequence of operation of the toaster for consecutive toast cycles.
A control for obtaining a desired color of toast in toasters often comprises a simple timer or counter. Such control utilizes a resistor-capacitor (RC) oscillator to determine the basic timing of the control and have the advantage of being relatively low in cost. Further, the user may readily regulate the control to obtain a desired toast color by varying the resistive portion of the RC oscillator.
To provide temperature compensation (shorter toasting cycle times as a result of the toaster cavity being warm as a consequence of the operation of the toaster for consecutive toast cycles), the control may include either a resistor or capacitor whose resistance or capacitance respectively will vary with temperature as part of the RC oscillator. In this manner, as the temperature within the toast cavity increases, the length of time for the toasting cycle will decrease to obtain a given color of toast. The temperature within the toasting cavity may be measured directly; however, there are other techniques which may equally be employed.
It has been found that placing the temperature sensing components directly in the toasting cavity reduces the operating life of the component. On the other hand, by placing the sensor outside the cavity, the changes in the electrical characteristic of the sensor is only proportional to the actual cavity temperature and may not provide a control having the desired sensitivity and accuracy.
If a dropping resistor is used as part of the power supply for the control, the heat generated by the dropping resistor will increase while the control is on. This resistor may then be employed to heat a component instead of actually measuring the temperature within the toasting cavity. This technique has the advantage of not requiring an external sensor in the toasting cavity; however, it may not provide a good model of the toasting cavity's temperature at all times.
To produce a consistent toast color, one should provide voltage compensation in addition to temperature compensation. This is necessary because the heat generated by the toaster heaters is proportional to the square of the supply voltage and the supply voltage may vary, for example between 112 and 128 volts. While the temperature varying component used to provide temperature compensation may also provide some voltage compensation, the amount of voltage compensation obtained is usually insufficient for effective operation of the control. This problem occurs because temperature compensation requires that the temperature sensor be heated at the same rate as the temperature rises within the toasting cavity, while voltage compensation requires the sensor to be rapidly heated to a value proportional to the supply voltage. Accordingly, one sensor cannot adequately provide both types of compensation.
It has been found that an electronic resistance element, such as a negative temperature coefficient thermistor, can be effectively used to monitor the temperature of the toasting cavity for temperature compensation purposes. It has been further found that the location of the thermistor relative to the cavity in horizontally disposed toasters is extremely important for effective operation. It has also been determined that the size of the opening communicating the thermistor with the toasting cavity is important to enable the thermistor to accurately sense the temperature within the cavity. Finally, it has been found that using the combination of a temperature sensitive capacitor and resistor increases the accuracy of the control in sensing the actual temperature in the cavity.
Accordingly, it is an object of this invention to provide temperature in a toasting control without significantly increasing the circuit complexity and cost. It is a further object of the invention to provide a control having temperature and voltage compensation which is effective in operation due to the manner in which the temperature sensing portion of the control is mounted on the toaster.